Manhattan DA Robert Morgenthau, who prosecuted disgraced state Sen. Guy Velella, disclosed yesterday he’s investigating “whether any criminal offenses were committed” when the former legislator was sprung from Rikers Island.

Morgenthau’s involvement became public when he asked the City Council to postpone an inquiry into Velella’s early release by the Local Conditional Release Commission.

Velella accepted a one-year jail sentence in a plea bargain in which Morgenthau agreed not to object to Velella’s plea for an early release.

Morgenthau wrote in a joint letter with Department of Investigation Commissioner Rose Gill Hearn that “our offices are conducting a joint investigation into . . . whether any criminal offenses were committed during the proceedings of the LCRC.”

The letter warned that if the council issued subpoenas, it “could inadvertently grant immunity to individuals who might otherwise be subjects of criminal proceedings.”

Council Speaker Gifford Miller quickly agreed to the request, saying he has full confidence the DA was an “independent person” who would provide an impartial accounting.

Miller also said the “question of criminal conduct didn’t arise from DOI until we started to ask people in the Bloomberg administration to testify.”

After speaking with Morgenthau, Miller decided to proceed with a hearing into the Velella mess on Monday. But he said witnesses would be asked to testify voluntarily, not under subpoena.

Amy Ianora, one of four LCRC members who resigned under pressure, has agreed to show up, according to Miller spokesman Steve Sigmund. But when reached at her Brooklyn home, Ianora wouldn’t confirm her attendance.

“I’m sorry, I’m not at liberty to make any comments about the situation,” she said.

DOI Commissioner Hearn issued a statement saying she still’s worried the council hearing could interfere with the investigation.

Hearn also indicated investigators are conducting a much more far-reaching probe than was known.

In rejecting the council’s role, she noted DOI has access to “search warrants, use of the grand-jury process and transactional immunity.”

Mayor Bloomberg continued to express confidence no one in his administration would be implicated.

“There is absolutely no evidence that anybody in the administration had any contact whatsoever with this board on behalf of Guy Velella or anyone else,” he said.

Another head rolled at the LCRC late Thursday, when executive director Louis Gelormino, who also served as a $135,000-a-year counsel at the Probation Department, quit suddenly.

Gelormino joined the city payroll in 1994. He acknowledged his mentor is Mary Sansone, a Brooklyn Democratic activist who supported Rudy Giuliani, Bloomberg and Gov. Pataki. Campaign finance records show Gelormino contributed to five political campaigns since 1993, including those of Herman Badillo – one of 36 officials who wrote letters on Velella’s behalf.

Badillo told The Post he doesn’t know Gelormino.

“Our offices are conducting a joint investigation into, inter alia, whether any criminal offenses were committed during the proceedings of the LCRC.”